1. Field of the Invention
This invention relates to drilling and has particular, but not exclusive, application to core drilling.
2. Description of Prior Art
In core drilling an annular drill bit is employed and the core produced by this bit is recovered for examination. One type of core drill is such that the whole drill string must be withdrawn to recover a length of core. In another type the core is received within a core tube which forms part of a retractable assembly which can be drawn back through the drill string by means of a wire line without the necessity to remove the drill string from the hole.
The development of the wire line technique has greatly reduced the labor involved in drilling holes, in that many core lengths can be recovered without withdrawing the drill string. However it is still necessary to withdraw the whole drill string to examine and replace the cutting bit and when drilling in hard formations this procedure should be carried out quite frequently. If a drill bit is replaced before it has become worn too badly it will provide quite valuable salvage. However it has been found that, because of the work involved in withdrawing a complete drill string, the bits of wire-line drills are often operated until they are so badly damaged that salvage is negligible.